Ink jet printing method and printer

ABSTRACT

An ink jet or other printer serves to print for each row of input pixels, two superimposed rows of contiguous “super pixels,” each print pixel being capable of receiving print contributions from N super pixels. The super pixels are twice the width of the input pixels and one row of super pixels is offset by half a super pixel width from the next row of super pixels. Redundancy is thus provided against the loss of a print element. The effects of smoothing or an image are reduced by edge enhancement processes.

This is the U.S. national phase of International Application No.PCT/GB03/03767 filed Sep. 1, 2003, the entire disclosure of which isincorporated herein by reference.

BACKGROUND OF THE DISCLOSURE

1. Field of the Disclosure

The disclosure relates to printing and, in a particularly importantexample, to ink jet printheads.

2. Related Technology

There is a demand for digital printers having a printhead that extendsacross the full width of the printed page offering both high throughputand high print quality.

In an ink jet printer of this character, having the necessarily largenumber of closely spaced ink chambers and nozzles, there will always bea risk of failure of one or more nozzles, whether as a consequence of amanufacturing error or through nozzle blockage or other failures in use.

It will be possible to detect and discard manufactured printheads havingeven a single failed nozzle. However, because of the very large numberof nozzles in each printhead, and because of the sophistication of themanufacturing techniques, such quality control measures would likelylead to an uneconomic manufacturing yield.

In use of the printhead, failure of even a single nozzle can lead toperceptible print artifacts, because of the spatial correlation of theartifact as the printed substrate is indexed past the printhead.

SUMMARY OF THE DISCLOSURE

Accordingly, the disclosure provides improved methods of printing andimproved printheads that are able to conceal artifacts arising fromnozzle failures or other departures from standard print performanceacross a print row.

Accordingly, the disclosure provides a method of printing parallel rowsof contiguous pixels on a substrate indexed in a direction orthogonal tothe rows, comprising the steps of printing for each row of pixels Nsuperimposed rows of contiguous super pixels, each print pixel beingcapable of receiving print contributions from N super pixels, and eachsuper-pixel preferably being elongated in the row direction with anaspect ratio of N:1.

Advantageously, each of the N superimposed rows of contiguous superpixels is offset in the row direction with respect to each of the othersuperimposed rows, with the distance of said offset preferably being 1/Nof the dimension of the super pixel in the row direction.

Preferably, print data are received in the form of an array of printdata pixels and wherein the value of each super pixel is derived as aweighted sum of preferably at least three corresponding data pixels witheach super pixel preferably symmetrically disposed with respect to printdata pixels.

Advantageously, at least one of the weighting coefficients applied tothe corresponding data pixels in said weighted sum is negative.

In a preferred form, the printability of each super-pixel 10 ismeasured, and the contribution to those pixels covered by thatsuper-pixel is transferred wholly or in part to one or more othersuper-pixels from which those pixels are capable of receiving printcontributions in accordance with any measured deviation in printabilityof that super pixel.

Suitably, an error in printability is measured for each super pixel andwherein the determination of the value of each super pixel includes afunction of measured error in printability, with that function beingpreferably polynomial and including terms to at least the third power.

According to a further aspect, the disclosure provides an ink jetprinter having a plurality of ink chambers each provided with a nozzlearrangement, the plurality of ink chambers being arranged so as to printon a substrate a row of contiguous print elements, the nozzlearrangement of each ink chamber being such that the print elementassociated with that ink chamber is elongated in the row direction withan aspect ratio of at least 2:1.

Advantageously, at least two sets of ink chambers are provided, each setbeing arranged so as to print a row of contiguous print elements, therows of contiguous print elements printed by the respective sets of inkchambers being superimposed.

Suitably, the print elements of one set of ink chambers are offset inthe row direction with respect to the print elements of another set ofink chambers with the offset being preferably the reciprocal of theaspect ratio.

In still a further aspect, the disclosure provides a method of printinga representation on a print medium of an array of print data pixelscomprising the steps of distributing print data from said array of printdata pixels over an array of super pixels in a distribution functionsuch that each super pixel receives a print data contribution from atleast two print data pixels and each print data pixel contributes printdata to at least two super pixels; and forming print pixels on themedium such that each print pixel receives print contribution from atleast two super pixels.

Preferably, the at least two super pixels from which a print pixelreceives print contribution, receive print data contributions fromdifferent combinations of print data pixels.

Advantageously, each super pixel receives a print data contribution fromat least three print data pixels with the print data contributionpreferably varying in sign between said print data pixels.

Suitably, the method may further comprise the step of measuring theprint efficiency of each super pixel, with said distribution functionpreferably including the measured print efficiency.

In a preferred form of the method, the step of forming print pixels onthe medium such that each print pixel receives print contribution fromat least two super pixels comprises the steps at each print pixel ofdepositing ink in an amount determined by one of the super pixels fromwhich that print pixel receives print contribution and, while thatdeposited ink remains fluid, depositing ink in an amount determined byan other of the super pixels from which that print pixel receives printcontribution.

In yet a further aspect, the disclosure provides a printer comprising aninput port adapted to receive an array of print data pixels; a printarrangement for forming overlapping super pixels on a print medium and aprint processor adapted to distribute print data from said array ofprint data pixels over the super pixels in a distribution function suchthat each super pixel receives a print data contribution from at leasttwo print data pixels and each print data pixel contributes print datato at least two super pixels.

BRIEF DESCRIPTION OF THE DRAWINGS

The methods and printers will now be described by way of example withreference to accompanying drawings in which:

FIG. 1 is a schematic view of an ink jet printhead according to theprior art;

FIG. 2 is an end view of an ink jet printer according to one embodimentof the disclosure, with a nozzle plate removed for clarity;

FIG. 3 is a sectional side view of an ink jet printer shown in FIG. 2;

FIG. 4 is a perspective view of an ink jet printer according to anotherembodiment of the disclosure, with parts removed for clarity;

FIG. 5 is schematic view (similar in diagrammatic form to FIG. 1) of anink jet printhead according to an embodiment of the disclosure;

FIGS. 6 and 7 are diagrams illustrating the alignment of input datapixels, super pixels and pixels printed on a substrate; and

FIG. 8 is a diagram illustrating the performance of an ink jet printeraccording to an embodiment of the disclosure.

DETAILED DESCRIPTION

Referring initially to FIG. 1, an ink jet printhead has a first array ofink chambers 10 defined by a piezoelectric wall structure 12. A nozzleplate 14 secured to the wall structure 12 defines a nozzle for each inkchamber 10. This first array of ink chambers is shown as depositing inkdroplets 16 on an appropriate substrate.

Ink jet printheads of this general form are described for example inEP-A-0 277 703 and EP-A-0 278 590.

To increase the number of ink droplets that can be deposited for a unitlength of the print row, it has been previously proposed to provide asecond array of ink chambers 18 similarly defined by a piezoelectricwall structure 20 and having a nozzle plate 22 defining one nozzle perink chamber 18. This second array of ink chambers 18 is shown asdepositing ink droplets 24 on the substrate. In this way, it is possibleeffectively to double the print resolution as compared with the“intrinsic” resolution defined by the nozzle spacing in a single arrayof ink chambers.

Each ink chamber 10 may be formed as an elongate channel, which iscollinear with and shares the same ink supply ports, as an elongatechannel forming a corresponding one of the ink chambers 18. The parallelarray of ink channels is then angled to create the offset in the twosets of nozzles.

If one chamber or nozzle should fail (as marked schematically at X),there will be an unprintable pixel in the print row. Even though thenumber of ink drops per unit length of the print row may be high(perhaps 360 dpi), a single unprintable pixel may still produce avisually unacceptable artifact because of the spatial correlation ofthat artifact as the print substrate is indexed relative to theprinthead.

Referring now to FIGS. 2 and 3, a set of print chambers 10 is defined bya piezoelectric wall structure 12. In this case, the nozzle plate 14serves to define two nozzles per ink chamber 10. Each of the two nozzlesis of the same or similar dimensions as the single nozzle in the FIG. 1arrangement and the two nozzles of each chamber are arranged to form asingle ink droplet 30 on the substrate of around double the volume ofthe ink drop 16 in the FIG. 1 arrangement. Each drop is elongated in thedirection of the print row, having an aspect ratio of 2:1 and each drophas a length such that they are contiguous across the print row.

The chamber structure may be modified such that it provides narrowerwall structures 12 and a wider print chamber 10 to allow space for thetwo nozzles. The walls may be as thin as 25 μm without significant lossof activity.

The print head of FIGS. 2 and 3 is of a structure commonly known as an“end shooter.” As is known, for example from EP-A-0 277 703 incorporatedherein by reference, channels 10 are formed in a block 32 ofpiezoelectric material polarized in the direction of arrow 35. Theapplication of a electric field across electrodes 34 formed on oppositesurfaces of a side wall 36 causes the piezoelectric material of the sidewall to deflect in shear mode, thereby causing the ejection of an inkdroplet from a nozzle associated with the channel. The position of thenozzles within the chamber is depicted schematically and may or may notbe provided entirely within the channel. It is often possible for aportion of the nozzle to overlap the walls without a significant changein ejection characteristics.

The channel terminates with a nozzle plate 14, within which the nozzlesare formed—as depicted in FIG. 3 which is a sectional view taken alongthe longitudinal axis of the channel.

The print head of FIG. 4 is a structure commonly known as a “sideshooter.” Nozzles 38 are provided within a cover plate 37 and arelocated at a point which lies between the ends of the channels. Thereare two nozzles for each channel 10. Ink ports (not shown) are providedat either end of the channel to allow circulation of ink through theejection chamber. A print head of this type, but with just a singlenozzle is described in WO 91/17051. The nozzles are shown schematicallyand are not to scale.

As illustrated schematically in FIG. 5 (which is of the samediagrammatic form as FIG. 1 and which depicts the nozzle and channelarrangements of either the embodiment of FIGS. 2 and 3 or the embodimentof FIG. 4), a second set of ink chambers is provided and is againdefined by a piezoelectric structure 20 with a nozzle plate or coverplate 22 defining two nozzles per ink chamber 18. These two nozzlescombine to form ink drops 32 which similarly have a 2:1 aspect ratio andform a contiguous row.

The second set of ink chambers 18 may be located within a separate printhead to that containing the first set of ink chambers 10 or may be partof the same print head such as described in WO 00/29217.

The ink drops 30 from the first array of ink chambers are offset alongthe print row with respect to the ink drops 32 of the second set of thechambers 18 by half the pitch of the ink chambers. With thisarrangement, if there is a failure of a single chamber such as thatshown schematically at X, no pixel remains unprintable.

It is convenient to regard the elongated ink drops 30 and 32 as printing“super-pixels,” each pixel printed on the substrate receivingcontributions from up to two super-pixels. The printed pixel structureis depicted in FIG. 5 as units A, B, C, D of line 40. In the control anddrive arrangement for the printhead, provision is made to distribute thedesired print density for a particular pixel between the twosuper-pixels which contribute to that pixel. In a typical arrangement,the desired print density for a pixel—established on a suitablegreyscale—would be distributed 50% each to the two correspondingsuper-pixels. In the event that a failure of an ink chamber (or theassociated nozzles) is detected, the distribution of print density canbe switched so that each of the two pixels covered by the now missingsuper-pixel receive 100% of the desired print density from the othersuper-pixel which covers that pixel. This compensation for a missingsuper-pixel through variation in the greyscale of neighboringsuper-pixels will effect neighboring pixels. Such effects will generallybe far less noticeable than an unprintable pixel. In an improvement,steps are taken to add noise (either by subtracting or adding greylevels) to distribute the effects of the missing lines over one or moreneighboring superpixels and reduce the spatial coherence of theartifact.

Although the row of super-pixels 13 (being the odd-numbered super-pixels1, 3, 5 . . . ) are shown in FIG. 5 to be transversely separated fromthe row of super-pixels 32 (being the even-numbered super-pixels 2, 4, 6. . . ), this is for drawing convenience only. The two rows ofsuper-pixels are effectively super-imposed.

One approach to deriving the greyscale levels for the super-pixels fromthe greyscale pixel values received as input print data is as follows.

The greyscale value of each super-pixel is set as one quarter of the sumof the greyscale values for the two pixels covered by the super-pixels,thus:S ₁=(P _(A) +P _(B))/4 S ₂=(P _(B) +P _(C))/4

This processing will serve as a low-pass spatial filter of the printimage. In regions where this spatial filtering may have a noticeableeffect on the image, as for example an edge, it will be possible to varythe algorithm or to pre-emphasize the edge so that the filtering hasless noticeable effect

In one embodiment of this disclosure, a print test is conducted tomeasure the print rate at each super-pixel for a nominal full blackprint density. This information is then employed in a calibrationprocess which determines during future use of the printhead how thesuper-pixel greyscale values S₁, S₂, . . . are derived from the inputpixel greyscale values P_(A), P_(B), . . . .

Thus in a case where the greyscale value of a pixel would be shared50%-50% between two super-pixels, prior knowledge that one super-pixelis being printed less effectively than another may cause an alternativedivision to be made. In the case where one super-pixel is not beingprinted at all, a division 0%-100% can be made. If there is simply areduction in the printed weight of a super-pixel by reason of somemanufacturing variance, a distribution such as 50%; 75% may be suitable.

A further approach to derive greyscale levels for the super pixels fromthe greyscale values received as input print data while correcting forboth errors within the print heads and enhancing the edges is asfollows:

A full black reference image is printed and for each super pixel theoptical density is measured. An average optical density is calculatedand the error for each super pixel calculated using the equation:E _(p)=1−(OD _(p) /OD _(average))

Where E_(p) is the error; OD_(p) is the measured optical density ofsuper pixel p and OD_(average) is the calculated average optical densityacross the row of super pixels.

A distributed error is calculated from the equation:DE _(p)=2E _(p) ^(n) −E _(p+1) ^(n) −E _(p−1) ^(n)

where n is a value greater than 1 and chosen such that only gross errorsare distributed through this term. A value of 4 is appropriate.

The image data is input as a value between 0 (no image data) and 1, fullblack for each print data pixel. The grey level for each print datapixel is denoted by the term g.

Super pixels are defined as shown diagrammatically in FIG. 6. It will beseen that each print data pixel (represented as the input grey levelg_(p)) has symmetrically aligned with it a super pixel (represented as acalculated base grey level G_(p)). Each pixel has an aspect ratio of2:1, extending in the row direction a distance twice the dimension ofthe print data pixel. The super pixels are arranged in two rows with thesuper pixels aligned with even print data pixels in one row and thesuper pixels aligned with odd print data pixels in another.

An alternative arrangement can be identified (as shown in FIG. 7) inwhich the array of super pixels is not symmetrically aligned withrespect to the array of input data pixels.

The base level G for each super pixel is then calculated from theequation:G _(p)=(g _(p)+(g _(p+1) +g _(p−1))/2)/2

It has been mentioned above that an effect of distributing the inputprint data values over super pixels may be to soften the printedrepresentation in the row direction. If appropriate, edges in the printdata may be “pre-enhanced” so as to reduce the perceived effect of thissoftening. In the present example, this enhancement is convenientlyeffected by defining:Edge_(p) =EHF*(2g _(p) −g _(p+1) −g _(p−1))

EHF is an arbitrary value selected on the edge enhancement required; atypical value being around 0.5.

The error in the grey value is at each super pixel (arising from themeasured error and the distribution of that error over neighboring superpixels) is then calculated from the equation:G ^(Error) _(p) =G _(p)*(E _(p) −DE _(p) *EECF)

where EECF is an arbitrary edge error correction factor; a typical valuebeing around 0.5.

The print data for each super pixel are subsequently calculated from theequation:Print=G _(p)/2+G ^(Error) _(p)/2+Edge_(p)

The calculated print data are sent to the ejection channels to print therequired image.

As illustrated schematically in FIG. 6, the p'th printed pixel P on thesubstrate receives print contribution from two super pixels. In thearrangement where the two rows of pixels are printed in a single passusing two rows of ejection chambers, the ink from the two super pixelscombines to produce an optical density determined by the sum of theprint values for those super pixels.

This described arrangement has a number of advantageous features. If aparticular ejection chamber is inoperable (so that the measured errorE_(p)=1), the effect of the error distribution is to increasecorrespondingly the grey level of those super pixels in the other rowthat overlap with the “failed” super pixel. There is therefore avoidedthe highly visible artifact of a straight line of unprintable pixels.This is illustrated diagrammatically in FIG. 8.

The softening that would otherwise accompany such redundancy in theprint capability is reduced by an edge enhancement process, ingeniouslyimplemented by adding negative terms to the weighted sum of input datapixels from which the print values for the super pixels are calculated.

Less gross errors detected by the measurement process are of course alsocompensated. The use of both linear and polynomial terms to provide forthis compensation of measured super pixel “printability” is of courseonly one of a number of alternative approaches. A gross error (typicallyarising from a failing ejection chamber) might be detected bythresholding the measured error and substituting an alternative errordistribution function, if that threshold is exceeded.

The calculations described above represent only one example of atechnique for distributing input print values over super pixels. Incertain applications, the step of measuring the printability of superpixels may be omitted. In other applications, nonalgebraic techniquesmay be employed. Also, the distribution may be caused to vary with theinput print data, if thought appropriate.

In other alternative arrangements, a number of super-pixels greater thantwo may contribute to each pixel. Thus, an arrangement having threearrays of print chambers, with each super-pixel covering three pixelsand with each pixel receiving print contributions from threesuper-pixels, can also be employed. This arrangement would be expectedto increase the resilience to super-pixel failures at the price ofincreased spatial filtering. In this case, with N=3 (rather than N=2, asin the previously described embodiments), the offset between rows ofsuper pixels can be 1/N of the dimension of the super pixel in thatdirection. In other arrangements, there may be no offset betweensuperimposed rows of sub pixels.

Experiments have shown that with piezoelectric operated ink jet printheads, it is possible to double the number of nozzles in an ink channelwith only a modest increase in the actuation voltage required. Ifnecessary, the nozzles formed in the applied nozzle plate may overhangthe piezoelectric wall structure to a certain degree withoutdramatically impairing the operation. The skilled man will recognizethat there are many alternative techniques for printing an elongatesuper-pixel having an aspect ratio of 2:1, 3:1 or greater. In certainapplications of the present invention, super pixels may have an aspectratio of 1:1.

In the description of preferred embodiments, the example has been takenof an ink jet printer with N rows of ejection chambers extending (orscanned) across the print medium, with the medium indexed in a directionorthogonal to the row direction after each pass. In an alternative, thesuper pixels from one pass are superimposed with super pixels fromanother pass. Care should be taken that N super pixels contribute toeach print pixel; this can be achieved—for example by ensuring that allN super pixels are printed while the ink remains wet or (in the case ofcurable inks) un-cured.

Concepts here described will find application in other printarrangements. In particular, the method of printing a representation ona print medium of an array of print data pixels comprising the steps ofdistributing print data from said array of print data pixels over anarray of super pixels in a distribution function such that each superpixel receives a print data contribution from at least two print datapixels and each print data pixel contributes print data to at least twosuper pixels; and forming print pixels on the medium such that eachprint pixel receives print contribution from at least two super pixels,will find useful application in arrangements other than ink jet printingand in arrangements other than a one dimensional printhead indexedacross a print medium.

1. A method of printing a representation on a print medium of an arrayof contiguous print data pixels comprising a plurality of parallel rowsof contiguous print data pixels, the method comprising the steps of:processing print data from said array of contiguous print data pixelssuch that it is distributed over an array of super pixels, each superpixel having a print level, according to a distribution function suchthat the print level of each super pixel is calculated based on a printdata contribution from at least two print data pixels and each printdata pixel contributes print data to the calculation of print levels forat least two super pixels; and forming print pixels on the medium suchthat each print pixel receives a print contribution from N (where N isan integer greater than 1) super pixels, wherein said processing stepcomprises distributing the print data for each of said rows of printdata pixels over a respective group of N superimposed rows of contiguoussuper pixels, and wherein each of said superimposed rows of super pixelsextends in a row direction and each row within a group of N rows ofsuper pixels is offset in said row direction with respect to each of theother superimposed rows in that group.
 2. A method according to claim 1,wherein the distance of said offset is 1/N of the length of each superpixel in the row direction.
 3. A method according to claim 1, whereinthe distance of said offset is 1/N of the length of each super pixel inthe row direction.
 4. A printer comprising an input port adapted toreceive an array of print data pixels, said array comprising a pluralityof parallel rows of contiguous print data pixels; a print arrangementfor forming overlapping super pixels on a print medium and a printprocessor adapted to process print data from said array of print datapixels such that it is distributed over the super pixels according to adistribution function such that the print level of each super pixel iscalculated based on a print data contribution from at least two printdata pixels and each print data pixel contributes print data to thecalculation of print levels for at least two super pixels; wherein eachprint pixel receives a print contribution from N (where N is an integergreater than 1) super pixels, wherein said print processor is furtheradapted to distribute the print data for each of said rows of print datapixels over a respective group of N superimposed rows of contiguoussuper pixels, said print arrangement being adapted to form said groupsof N superimposed rows of contiguous super pixels, and wherein each ofsaid superimposed rows of super pixels extends in a row direction madeach row within a group of N rows of super pixels is offset in said rowdirection with respect to each of the other superimposed rows in thatgroup.
 5. A printer according to claim 4, adapted to form N super pixelsfor each print pixel.